山西吉县沃曲桃园下三叠统刘家沟组红层的古地磁研究

本文对鄂尔多斯盆地东南缘一个背斜剖面的早三叠世红层样品进行了古地磁研究.对逐步热退磁矢量序列进行主成分分析以及各磁组分的解阻温度谱分析,估计了携磁矿物.分离出4种磁成分:镜铁矿携带的沉积或沉积后剩磁;赤铁矿携带的化学剩磁;等温剩磁和粘滞剩磁.特征剩磁（最高解阻温度T_u≥670℃）通过倒转检验、（递增）褶皱检验,平均方向为偏角D=-25°,倾角I=41°,相应的极位置为65°N、356°E.     

Paleomagnetism of the Upper Silurian and Lower Devonian carbonates of New York State: evidence for secondary magnetizations residing in magnetite

Paleomagnetic directions for the Upper Silurian and Lower Devonian carbonates of the Helderberg escarpment (New York State) differ from expected Late Silurian and Early Devonian directions for cratonic North America. The mean direction (D = 165°, I = −10°; paleopole at 50°N 129°E) is similar to Late Carboniferous and Early Permian results. Negative fold tests, and a lack of reversals, suggest that the magnetization is secondary. However, low coercivities, low blocking temperatures, the thermomagnetic curves (T_C near 570°C) and the acquisition of isothermal remanent magnetizations all suggest that the remanence is carried by magnetite. If a detrital origin of these magnetites is assumed, the secondary nature of the remanence would argue for thermal resetting as a result of deep burial of the rocks. However, no evidence for such thermal resetting is seen in the alteration of conodonts. More likely perhaps is a chemical or thermochemical origin of the remanence; this would require the magnetites to be authigenic.     

Multiphase magnetizations: Problems and implications

Experiments combining different cleaning and analytical techniques indicate that multiphase magnetizations may be quite common. However, these may not be recognized because of limited experimental work. Alternating field (AF) cleaning is often the only treatment applied to igneous and metamorphic rocks; thermal and/or AF cleanings are usually the only treatments applied to sedimentary rocks. In many instances, AF and thermal treatments are limited to 100 mT and 550°C respectively. Investigations based on such limited cleaning often fail to detect some of the phases of magnetization contained in the rock. Failure to detect one or more phases means that valuable data are not recovered and the whole magnetic history of the rock has not been unfolded. Most importantly, the undetected phase might be the initial so that a secondary magnetization can easily be mistaken for the initial with an erroneous interpretation as a result. It is therefore imperative to recognize all phases of magnetizations and it possible to separate them.Procedures that have been used to recognize and unravel multiphase magnetizations are described. These procedures make use of chemical, thermal and AF cleaning treatments, two-stage demagnetization, vector analysis, slicing of specimens and vector diagrams. The combination used depends on the rock studied. For example, it is found that AF followed by thermal treatment can be very useful for the study of igneous rocks; chemical leaching is by far the most effective cleaning technique for the study of red beds. A three-phase model describing the magnetizations process of certain red beds is proposed. The slicing of specimens is used to explain intermediate directions with respect to field reversals. It is shown that graphical representation by vector diagrams can greatly facilitate the interpretation of the results. The examples show that, although a statistically well defined magnetization may be obtained after limited cleaning, it cannot be assumed to be the initial. One must ascertain that another magnetization has not remained undetected. This necessitates detailed and extensive experimental work using and devising new analytical procedures in an attempt to unfold the whole magnetic history of the rock. It is noted that tentative polar paths constructed from results obtained after inadequate experimental work cannot be up-graded by adding more data points of doubtful or unproven quality. The evolutionary process of polar paths is dependent upon increasing the reliability of palaeomagnetic results.     

Paleomagnetism of mid-Cretaceous red beds in west-central Kyushu Island, southwest Japan: paleoposition of Cretaceous sedimentary basins along the eastern margin of Asia

A paleomagnetic study was carried out on the mid-Cretaceous sedimentary strata in west-central Kyushu Island, southwest Japan, to elucidate the origin of sedimentary basins along the Asian continental margin in the Cretaceous. We collected paleomagnetic samples from a total of 34 sites of the mid-Cretaceous Goshonoura Group, shallow-marine clastic deposits in west-central Kyushu, and characteristic remanent magnetizations were recognized from 18 horizons of red beds. Thermal demagnetization has revealed that the red beds contain three magnetization components, with low (<240°C), intermediate (240-480°C), and high (480-680°C) unblocking temperatures. The low unblocking temperature component is present-field viscous magnetization, and the intermediate one is interpreted as chemical remanent magnetization carried by maghemite that was presumably formed by post-folding, partial oxidation of detrital magnetite. Rock magnetic and petrographic studies suggest that the high unblocking temperature component resides largely in hematite (martite and pigmentary hematite) and partly in maghemite. Because of the positive fold test, this high temperature component can be regarded as primary, detrital remanent magnetization. The tilt-corrected mean direction of the high temperature component is Dec=65°, Inc=63° with α₉₅=5°, which yields a paleomagnetic pole at 39°N, 186°E and A₉₅=8°. A combination of this pole with those of the Late Cretaceous rocks in southwest Japan defines an apparent polar wander path (APWP), which is featured by a cusp between the Late Cretaceous and the Paleogene. A comparison of this APWP with the coeval paleomagnetic pole from northeast Asia suggests an approximately 50° post-Cretaceous clockwise rotation and 18±8° southward drift with respect to northeast Asia. The southward transport of the Cretaceous basin suggests that the proto-Japanese arc originated north of its present position. We propose that the coast-parallel translation of this landmass was caused by dextral motion of strike-slip faults, which previous geodynamic models interpreted to be sinistral through the Mesozoic. The change in strike-slip motion may have resulted from Mesozoic collision and penetration of exotic terranes, such as the Okhotsk microcontinent, with the northeastern part of Asia.     

Magnetic components contributing to the NRM of Middle Siwalik red beds

The components of the NRM in Middle Siwalik red beds are carried by two phases of hematite, a red pigment phase and a specular hematite phase. We present evidence in the form of a conglomerate test that the specularite phase carries a remanence acquired during or shortly after deposition. The red pigment, however, post-dates deposition, in many cases by at least one reversal boundary. This secondary component has a higher coercivity but a lower blocking temperature in these rocks and can therefore be selectively removed by thermal demagnetization to reveal a primary magnetization useful for magnetostratigraphic studies.     

The A.R.M. correction method of paleointensity determination

A new method has been developed for recovering paleointensities from igneous rocks which requires only a single heating to the Curie temperature, utilizing partial ARM acquisition measurements before and after the heating to carefully separate distortions of the coercivity spectrum, caused by the heating, from viscous components present in the natural remanence. In addition, the changes in the partial ARM curves may be used to correct to some extent for chemical alterations that occur during the heating. As a result, the method allows one to use a number of samples for paleointensities that would normally be rejected by the Thellier-Thellier method, especially those contaminated by strong viscous overprints. The key to this process is the high sensitivity to changes in the TRM and ARM acquisition spectrum (and hence the coercivity spectrum) of partial ARM acquisition curves, which may be obtained at room temperature.Paleointensity results from recent historic Hawaiian lava flows have demonstrated the reliability of the ARM correction method.     

Some rock magnetic properties of mid-Cretaceous basalts from Israel and India (Rajmahal traps), and their bearing on palaeointensity experiments

A wide range of rock magnetic properties have been determined from two collections of mid-Cretaceous basalts; one from Israel, the other from the Rajmahal traps in northeast India. Deuteric oxidation is rare in both collections, with titanium-rich titanomagnetite being the principal remanence carrier in most cases. There are a number of differences in rock magnetic properties between the two groups. Some of these seem to be primary, whereas others appear to be caused by hydrothermal alteration and weathering, which are more prevalent in the Indian rocks. These rocks are being used in palaeointensity experiments, from which it is hoped to determine the strength of the Earth's magnetic field during the long period of normal polarity in the mid-Cretaceous. Thellier palaeointensity experiments have been performed on two samples from each site. The degree of agreement between the two results is highly variable. The low blocking temperatures and the presence of secondary viscous components in many samples make Thellier palaeointensity experiments very difficult. A further problem is that of thermal alteration, two main types of which are observed. The first manifests itself as a large and sudden increase in partial thermoremanent magnetization (pTRM) capacity, and the second as a steady decrease in the size of pTRM with increasing temperature.     

基于剩磁各向异性方法对华北下三叠统红层磁倾角浅化效应的研究

本文报道利用岩石剩磁组构对华北下三叠统红层进行磁倾角浅化效应的进一步识别与校正研究结果.首先,采用45°等温剩磁各向异性方法,即通过沿与样品原始水平面(即层面)呈45°夹角方向施加磁化场获得等温剩磁,并进行逐步热退磁,获得平行于层面和垂直于层面的等温剩磁分量随外加磁场和热退磁温度的变化趋势,计算获得浅化因子f=0.70.其次,应用高场等温剩磁各向异性方法,结合峰值为100 mT的交变退磁和120℃热退磁处理,分离获得碎屑赤铁矿对剩磁各向异性的贡献;由直接测量获得的单颗粒碎屑赤铁矿的各向异性度(a=1.35),计算获得f=0.59.该结果与前人对刘家沟组红层进行E/I法磁倾角浅化校正的结果(f=0.60)具有很好的一致性;表明华北下三叠统刘家沟组红层磁倾角浅化效应显著,其浅化因子为f=0.59;高场等温剩磁各向异性方法是红层磁倾角浅化校正的最有效方法.同时,如果有足够的独立样品数,且特征剩磁来自于单一剖面或有证据表明多条采样剖面之间未发生显著的相对运动,E/I法对红层磁倾角浅化因子的估计也是可信的.     

闽西晚白垩世红层的古环境探究

中国东南地区白垩纪红层通常被认为是陆相红色碎屑沉积物,是河流湖泊相沉积,但其古环境存在争论.本文以闽西晚白垩世红层为研究对象,利用环境磁学、粒度、地球化学、古土壤分析等方法,选取连城(LC)和冠豸山(GZS)两个剖面,分析探讨红层的古环境.结果显示:(1)闽西红层主要以细的粉砂颗粒为主,黏土和砂含量较少,表现为粉砂和砂互层,夹有薄层细粒砂砾层;样品磁化率偏低,主要载磁矿物为硬磁性矿物赤铁矿.(2)闽西红层有较高的风化程度,是暖湿气候下的中等风化程度;轻重稀土元素分异明显,Ce元素富集,Eu元素相对亏损,与上地壳(UCC)的分配模式相似,表明沉积物经历了充分混合,物质为混合沉积产物.(3)闽西红层虽然发育了一定的古土壤,但是成壤程度不强,没有明显的古土壤粘化层(Bt)和钙积层(Bk).由此推断,闽西红层沉积物在沉积前已在源区经过相当程度的风化过程,之后经过混合动力搬运沉积在盆地,沉积后风化成壤弱.可见,红层本身并不能直接反映沉积区环境,需结合古土壤发育特征判断沉积环境特征和变化.(4)闽西晚白垩世红层表现为相对干旱半干旱的古环境,红层中主要的着色矿物赤铁矿主要形成于源区,反映了地表透水性良好的干燥氧化条件,而不是"水成"环境.本文可为白垩纪红层古环境研究提供新的思路.     

Chemical magnetic overprinting in the Ordovician redbeds of Buçaco (Portugal)

Cambro-Ordovician redbeds have been sampled in the Buçaco syncline, an Hercynian structure of northern Portugal. Secondary and characteristic components of magnetisation have been recovered for samples from seven sites on the same limb of the syncline using thermal and chemical demagnetisation techniques, suggesting both pre- and post-tectonic magnetisations. The secondary magnetisations include: (1) the present Earth field direction with normal and reverse polarity, interpreted as a stable Quaternary magnetisation; and (2) a group with only normal polarities, of constant inclination (50°) but variable declination (320–350°). The mean inclination of this second group suggests a Cretaceous origin, and acquisition of magnetisation during the opening of the Bay of Biscay could explain the data.There are two periods of sedimentation after deposition of the Cambro-Ordovician redbeds that correspond to the acquisition of the secondary remanent magnetisations. We propose a process involving the fossilisation of alteration products immediately preceding sedimentation in order to explain this correlation.     

Frasnian-Famennian boundary events in Tarim Basin,Xinjiang, Northwest China

The Frasnian-Famennian Boundary Event (Kellwasser Event) is one of the seven strongest Phanerozoic faunal turnovers. The importance of this first order of global bio-event was taken into consideration when the International Subcommission on Devonian Strat…     

Experimental modeling of the chemical remanent magnetization and Thellier procedure on titanomagnetite-bearing basalts

The results of the experimental studies on creating chemical and partial thermal remanent magnetizations (or their combination), which are imparted at the initial stage of the laboratory process of the oxidation of primary magmatic titanomagnetites (Tmts) contained in the rock, are presented. For creating chemical remanent magnetization, the samples of recently erupted Kamchatka basalts were subjected to 200-h annealing in air in the temperature interval from 400 to 500°С under the action of the magnetic field on the order of the Earth’s magnetic field. After creation of this magnetization, the laboratory modeling of the Thellier–Coe and Wilson–Burakov paleointensity determination procedures was conducted on these samples. It is shown that when the primary magnetization is chemical, created at the initial stage of oxidation, and the paleointensity determined by these techniques is underestimated by 15–20% relative to its true values.     

河南灵宝盆地中新生代陆相地层的新划分与对比研究

灵宝盆地位于华北板块南缘与秦岭造山带之间,是豫西北一系列北东-南西向断陷盆地之一.盆地内沉积地层主体为一套厚约2000m陆相碎屑岩夹泥灰岩、薄煤层沉积.本文根据盆地内的恐龙蛋、介形虫、腹足类、哺乳动物化石及少量孢粉等,将地层自下而上划分为下白垩统枣窳组、上白垩统南朝组、古近系古新统-下始新统项城群、中始新统川口组、上始新统庄里坡组及新近系上-中新世（组名暂未定）等6个地层单元.研究表明：下、上白垩统之间及其与项城群之间为不整合或超覆,上中新统与川口组或庄里坡组为不整合接触,整个地层是一套河流相沉积、局部洪泛洼地或小浅湖相沉积.     

Paleomagnetic evidence for the relative positions of Timor and Australia in the Permian

An Upper Permian paleomagnetic pole has been determined for the Cribas Formation in eastern Timor. The co-ordinates for the mean pole are 159.8°E and 56.6°S,α₉₅ = 9.0. The reliability of the pole is ascertained through thermal demagnetization, a fold test, comparison between red beds and a lava flow, and the presence of normal and reversed polarities. The Timor pole is in excellent agreement with the Australian Upper Permian and Triassic poles. From this it is inferred that autochthonous Timor formed part of the Australian continental margin at least since the Upper Permian.     

Small channels: Their experimental formation,nature, and significance

The manner in which small channels are generated, from plane beds beneath sheet flows, has been experimentally elucidated. On plane, erodible, sand beds, the transition from thin, supercritical sheet flows to the channelled condition was studied over ranges of discharge, slope, and temperature. Secondary flow of the second kind, its action facilitated by steep vertical velocity gradients in the primary flows, caused sheet-flow instability. Along junctions between neighbouring secondary cells, both either raised or lowered elements of the primary flow. In the latter case, fast surface water was lowered to the bed, causing relatively intense, local, longitudinal scour. Dislodged grains were moved divergently to either side, leaving straight, central trenches. Development of positive feedback between cells and trenches led to rapid enlargement of the latter and concomitant growth of paired levees. The resulting structures, ‘protochannels’, were themselves ephemeral, developing two types of instability associated with secondary flow of the first kind. Firstly, small deviations from bilateral symmetry were enhanced, causing evolution into meandering channels. Secondly, headcutting led to multiple tributary development and, at resulting confluences, the action of strong pairs of secondary cells led to the development of braiding channels. Because they are shortlived, protochannels are but rarely seen in nature. Their seeding is markedly temperature-sensitive, reflecting their frictional origin. The erosive power of shallow overland flow largely depends on flow-energy concentration by secondary flow, firstly into channels, then within the channels themselves. Suppression of secondary flow, as by intense raindrop bombardment, can stabilize sheet flows. In deeper water, the effects of secondary flow appear relatively less dramatic. However, even if such motion is weak, bedload divergence from attachment lines can favour entrainment locally and thus affect bed geometry. Analogy between our results and river behaviour appears close and. on continental shelves where water must often flow as sheets, structures resembling giant protochannels evidently persist.     

Impacts of river bed gas on the hydraulic and thermal dynamics of the hyporheic zone

Despite the presence of gas in river beds being a well known phenomenon, its potential feedbacks on the hydraulic and thermal dynamics of the hyporheic zone has not been widely studied. This paper explores hypotheses that the presence of accumulated gas impacts the hydraulic and thermal dynamics of a river bed due to changes in specific storage, hydraulic conductivity, effective porosity, and thermal diffusivity. The hypotheses are tested using data analysis and modelling for a study site on the urban River Tame, Birmingham, UK. Gas, predominantly attributed to microbial denitrification, was observed in the river bed up to around 14% by volume, and to at least 0.8 m depth below river bed. Numerical modelling indicates that, by altering the relative hydraulic conductivity distribution, the gas in the river bed leads to an increase of groundwater discharge from the river banks (relative to river bed) by a factor of approximately 2 during river low flow periods. The increased compressible storage of the gas phase in the river bed leads to an increase in the simulated volume of river water invading the river bed within the centre of the channel during storm events. The exchange volume can be more than 30% greater in comparison to that for water saturated conditions. Furthermore, the presence of gas also reduces the water-filled porosity, and so the possible depth of such invading flows may also increase markedly, by more than a factor of 2 in the observed case. Observed diurnal temperature variations within the gaseous river bed at 0.1 and 0.5 m depth are, respectively, around 1.5 and 6 times larger than those predicted for saturated sediments. Annual temperature fluctuations are seen to be enhanced by around 4 to 20% compared to literature values for saturated sediments. The presence of gas may thus alter the bulk thermal properties to such a degree that the use of heat tracer techniques becomes subject to a much greater degree of uncertainty. Although the likely magnitude of thermal and hydraulic changes due to the presence of gas for this site have been demonstrated, further research is needed into the origins of the gas and its spatial and temporal variability to enable quantification of the significance of these changes for chemical attenuation and hyporheic zone biology.     

Red beds: DRM or CRM?

Thermal demagnetization and chemical leaching experiments show that, in some Hopewell Group sediments, both detrital (DRM) and chemical (CRM) remanent magnetizations are present. The CRM was acquired in two phases (CRM_A, CRM_B), one before and one after a field reversal. Field evidence indicates that all three magnetizations were probably acquired in less than 35 my. The magnetization process occurred over a period of time long enough to span one or more field reversals. The CRM_B and then the CRM_A can be successively removed by progressively longer immersion times in hydrochloric acid so that the DRM is uncovered. The oppositely directed CRM_A and DRM cannot be thermally separated so that accurate field directions cannot be determined from thermal demagnetization alone. It is suggested that field reversals occurringduring magnetization may have a bearing on different paleomagnetic problems. The two techniques of thermal demagnetization and chemical leaching complement each other and together they can provide new information about the behaviour of the earth's field in the past.     

The Application of Combined Seismic and Electrical Measurements to the Determination of the Hydraulic Conductivity of a Lake Bed

The hydraulic properties of lake beds control the interactions between lakes and ground water systems, but these properties are normally difficult to measure directly. The authors'method combines seismic reflection and electrical measurements to map the relative hydraulic conductivity of lake bed sediments. A shipboard seismic profiling system provides sediment thickness, while a towed electrical array yields longitudinal conductance and electrical chargeability. The sediment's leakance (hydraulic conductivity/thickness) can be calculated from the longitudinal conductance data. Leakance may then be converted to relative hydraulic conductivity through the seismically derived sediment thicknesses. Simultaneously acquired electrical chargeability provides an independent measure of clay content. The seismic and electrical systems are computer automated and yield production rates of approximately five line-kilometers/hour or 300 electrical soundings/hour. The systems provide continuous hydraulic information along the ship track rather than the point information derived from coring.
The procedure and systems have been used to map the bed of Lake Michigan offshore from an area of heavy pumpage. This location has been chosen to test the method because lake water has intruded the aquifer in plumes largely controlled by lake bed hydraulics. Mapping these plumes onshore permits the inference of the spatial distribution of offshore hydraulic conductivities. Offshore seepage measurements and numerical, chemical transport modeling of this site have confirmed the reliability of the geophysically derived hydraulic conductivities and have also demonstrated the improvement in numerical results achieved through the availability of spatially determined hydraulic conductivities.     

A wind tunnel investigation of particle segregation,ripple formation and armouring within sand beds of systematically varied texture

This paper reports on a wind tunnel investigation of particle segregation, ripple formation and surface armouring within sand beds of systematically varied particle size distribution, from coarsely skewed to bimodal. By design, the system was closed with no external inputs of mass from an external particle feed. Particles too coarse to travel in saltation for the given range in wind speed were dyed red in order to distinguish them in optical images from finer sand particles, which could be entrained into the unidirectional airflow. A 3D laser scanner measured the changing bed topography at regular time intervals during 18 experiments involving varied combinations of wind speed and bed texture. Image classification techniques were used to investigate the coincident self‐organization of the two populations of particles, as distinguished by their colour. As soon as saltation commenced, some of the red particles segregated into thin discontinuous patches. Particle trapping and sheltering on these rough patches was strongly favoured, causing them to grow preferentially. During the earliest stages of formation, bedform growth coincided with: (i) rapid coarsening of the surface texture; and (ii) the merging of proto‐ripple ‘crests’ to generate larger rhythmic bedforms of lower frequency. Consistent with previous work, ripple size was observed to increase under stronger winds when not exceeding the threshold for entrainment of the coarse‐mode or red particles from the crest. With declining rates of mass transport and particle segregation as the bed surface armoured, and the consequent deceleration of ripple propagation through to the end of each experiment, all surfaces eventually attained a steady‐state morphometry. At saturation, the largest ripples developed on beds having the lowest initial concentration of red particles. Copyright © 2016 John Wiley & Sons, Ltd.     

Origin of cyclicity in Triassic-Jurassic radiolarian bedded cherts of the Mino accretionary complex from Japan

Abstract The abundance of magnetic microspherules in a Triassic-Jurassic continuous sequence of alternating chert and shale beds in the Mino accretionary complex, central Japan, was measured systematically. Depending on time, the magnetic microspherules extracted from shale beds change in abundance considerably from the minimum 0.9ppm/cm³ at latest Triassic ( ca 208Ma) and the maximum 75ppm/cm³ at late Early Jurassic ( ca 187Ma); however, the abundance is always higher approximately 10–100 (average 70) times than those from adjacent chert bed at any stratigraphic horizon. Such systematic difference reveals the origin of radiolarian bedded chert as cyclic-rapid accumulation of biogenic SiO₂ under extremely slow accumulative environments of shale with probable aeolian dust in origin. The accumulation data for individual shale and chert beds were obtained based on the microspherule abundance and radiolarian biostratigraphy, i.e., ca 0.018g/cm²Ka for lower Jurassic shale beds and ca 1.9g/cm²Ka for adjacent chert beds.
Duration time to make a chert-shale couplet corresponds to a dominantly 15–20Ka interval (average 23 Ka) in Upper Triassic bedded cherts with a low paleolatitude, whereas a 40–45 Ka interval (average 42 Ka) in Lower Jurassic ones which may been formed in higher latitude than Triassics before the final accretion to the Asian continental margin. Depending on paleolatitude, the cyclicity of 23 and 42 Ka may correspond to Milankovitch cycles which have been well documented in deep-sea sediments.